Mechanical tuning for magnetrons



Sept 3, 1957 J. KLINE 2,805,362

MECHANICAL TUNING FOR MAGNETRONS Filed Aug. 12, 1954 2 Sheet-Sheet 1 P/GJ /9 Maj;

Sept 3, 1957 J, iN 2,805,362

MECHANICAL TUNING FOR MAGNETRONS Filed Aug. 12, 1954 2 Sheets-Sheet 2 FIG. 5 F76. 4 O

2840 E Q o E 3 33/0 3 E 3 53,970 g b i g I 4,960

60 O 255 07/0 22.65 30.20 3275 +5.50 52.85 cm www 63 5,

2; 62 M VEWTOA JAG/r [LINE fun/Q6 5% a %-6/ 8y T F? flrrae 5y ilited Sttes MECHANICAL TUNHJG F OR MAGNETRUNS Jack Kline, Concord, Mass, assignor to Raytheon Manufacturing Company, Waltham, Mass, a corporation of Delaware Application August 12, E54, Serial No. 449,397

10 Claims. (Cl. 315---39.61)

This invention relates to mechanical means for tuning a magnetron.

In some applications of multicavity magnetrons it is desirable to be able to tune the oscillator over a wide range of frequencies. This can be done by the known capacitive and inductive tuners. However, when the magnetron anode is formed with many small cavities or when a complicated cavity geometry is used such conventional tuning structures are no longer practical.

The desired broad band tuning range can be achieved in such cases by extending the anode structure of a vaned or multicavity anode beyond one end of the cathode in the axial direction and positioning a structure within the vane tips for axial movement. This structure operates as a noncontacting short across the wave guide considered as formed by the extended anode. The tuning structure can be formed of conductive material as a solid cylinder, or such a cylinder with one or more enlargements. The noncontacting nature of the short requires that some means'be provided to prevent energy escaping into the space beyond the short. Coatings of a lossy material applied beyond the short prevent the leakage of energy and thus the development of undesired modes of oscillation. In addition to tuning over a broad band of frequencies, the structure is relatively simple to design and build. It does not introduce interfering modes of operation to the magnetron. Its tuning curve can be quite closely approximated by a mathematical formula. The simplicity of the shape of the tuning structure makes it easier to apply cooling structures to it than to some capacitive tuners.

Other and further objects and advantages of this invention will become apparent as the description thereof progresses, reference being had to the accompanying drawings wherein:

Fig. 1 is a section along the line 1-1 of Fig. 2 of a magnetron embodying the invention;

Fig. 2 is a section along the line 2-2 of Fig. 1;

Fig. 3 is a schematic diagram of a shorted transmission line, the approximate equivalent of the electrical circuit of the invention;

Fig. 4 is a schematic diagram of such a transmission line shunted by an inductance, a closer approximation of the electrical circuit of the invention;

Fig. 5 shows a series of tuning curves for a tunable magnetron of the type shown in Figs. land 2; and

Fig. 6 is a somewhat schematic diagram of a modification of the invention.

In Figs. 1 and 2 the reference numeral 10 designates the anode shell of a magnetron with radial vanes 11 forming a plurality of resonant cavities. A cathode 12 is concentrically located within the vanes 11 and mounted in an opening in the lower pole piece 13 that is fixed in an opening in the lower cover plate 14. An output coupling 15 of any of the well-known designs is mounted in an opening in the outer circular wall of the anode 10 and protrudes into one of the cavities. It will be noted that, the vanes 11' extend up beyond the cathode 12. A

ICC

cylindrical plunger 16 formed with two enlarged sections 17 and 18 with a central reduced section 20 between them is centrally located within the space defined by the tips of the vanes 11. A threaded extension 21 of the plunger 16 is brought up through an opening in the upper pole piece 22 set in an opening in the upper cover plate 23. A nut 19 is screwed on the threaded portion of the plunger 16 that extends from the pole piece 22. This extension 21 is formed with a shoulder 24 to which a diaphragm'25 is attached. The diaphragm is attached at its outer edge to a shoulder 26 formed on the upper cover plate 23. Any convenient structure that permits mechanical movement through a vacuum tight seal can be used for this purpose. The upper pole piece 22 and the lower pole piece 13 are connected to the magnet 21 to complete the magnetic circuit through the reaction space of the tube. Two straps 28 and 30 are mounted in notches 31 in the bottom of the vanes 11 in the wellknown manner.

The operation of the structure is best understood by regarding the extended anode 10 with its vanes 11 as a wave guide or other transmission line with radio frequency energy propagated axially along it as shown schematically in Fig. 3. The plunger 16 can be regarded as a noncontacting movable short 46 terminating the transmission line 41 to form a circuit resonant at a frequency determined by the geometry of the vanes and the plunger and the distance [1 of the lower face of the plunger, or shorting bar 40 from the upper face of the cathode 12 to form the end of the transmission line 41 as shown schematically in Fig. 3. Each anode cavity can be considered as a separate wave guide with a radial crosssection. If the resonant wavelength is so chosen that it is greater than the cutoff wave length of these smaller Wave guides formed by the vanes, and the length of the anode beyond the top of the cathode is long enough, the cavities will act like shunting inductances represented by coils 42 in Fig. 4 and these wave guides will have a virtually constant reactance for all positions of the plunger 16. The anode resonant frequencies and the wave guide wave length can be shown to be related as follows:

are the wave lengths at difierent modes.

2, etc.

A =free space wavelength at the resonant frequency. )t waveguide wavelength corresponding to a particular mode characterized by A At resonance the length of the anode above the cathode h equals in the case of anode cavities shorted at one end and 7\ =2h in the case of anode cavities open at both ends, substituting the first'valu'e in Equation 1 and rearranging which makes A =4h a t b 7 J The eds-9 of h st s 2? and 30 is to add ca ac ti e loading to the end of the wave guide. This added capacitive loading can be accounted for mathematically as a ance in t9 t e e f ct e ilengthhr r If is ta en s bein e t e t ic o fo i e e en hv as ind cat d b e, t u n r can be s o all do.

The recessed section 20 of the plunger 16 has the efiect ofa choker Itserves to isolate the region beyond the shunt from the region between the shunt and the interaction space tha't would otherwise have an effect on the resonant frequency.

The variation of frequency is shown in Fig. plotted vertically along the line 50 and a dimensional equal to twice the length of the effective portion of the extension of the anode structure beyond the reaction space, plotted horizontally along-the line'51. The curves 52, 53, 54 and 55 show the calculated relationship between the effective length of the extension for the fundamental Pi mode and higher Pi modes. The points 5211, b, e d, e, and f, 53 a, b, and c, 54 a, b, and 0, and 55 a and b are experimentally determined points from measurements madeby cold testing a representative magnetron having a tuning structure made in accordance with the invention. It will be noted that the relationship between the displacement of the plunger and the resulting frequency is approximately linear at higher frequencies for the fundamental Pi mode andd'less linear for the lower frequencies in the higher mo c.

"Fig. 6 shows another embodiment of the invention. A tuning plunger 60 is formed with a single enlarged section '61-. This plunger is shown suspended within the cylindrical space defined by the vanes 62 of the anode cavity 63. The restricted portion 64 of the plunger 60 has a coating 65 of a lossy material, preferably a cermct, that is, a mixture of ceramic and metallic materials which serves to cause'losses that present a termination to the active portion ofthe plunger. The impedance of this termination is relatively independent of the geometry of 212;; cavity beyond the effective portions 61 of the plunger This invention is not limited to the particular details of construction, materials and processes described, as many equivalents will suggest themselves to those skilled in the art. It is accordingly desired that the appended claims be given a broad interpretation commensurate with the scope of the invention within the art.

What is claimed l. A tunable electron discharge device comprising a cathode, an anode structure formed rescnant cavities arranged coaxially about said cathode and extending beyond said cathode, means adjacent said anode'strifcture for-producing a magnetic field in the space between said cathode and anode, a cylindrical piece'of conductive material, means to support said piece coaxially within said anode in the space beyond said cathode and to move said piece axially within said space. Q

2. A tunable electron discharge device comprising a cathode, an anode structure formed with resonant cavities arranged coaxially about said cathode and extending beyond said cathode, means adjacent said anode structure for producing a magnetic field in the space between said cathode and anode, a cylindrical piece of conductive material, formed with a portion having a reduced crosssectional area located between portions of larger cross area, means to support said piece coaxially within said anode in the space beyond said cathode and to move said piece axially within said space;

3. A tunable electron discharge device comprising a cathode, an anode'structure .formed with resonant cavities arranged coaxiallyaboutsaid cathode and extending beyond said cathode, means adjacent said anode structure is; roducin a nespet s fie d in th 59??? t en said cathode and anode, a cylindrical piece of conductive material, means to support said piece coaxially within said anode in the space beyond said cathode and to move said piece axially within said space, comprising a threaded extension on said piece mounted in said anode structure and a flexible diaphragm connecting said piece and said anode structure.

4. A tunable electron discharge device comprising a cathode, an anode structure formed with resonant cavities arranged coaxially about said cathode and extendmg beyond said cathode, means adjacent said anode structure for producing a magnetic field in the space between said cathode and anode, a cylindrical piece of conductive material formed with a portion having an increased crosssectional area in juxtaposition to the openings in the anode cavities, means to support said piece coaxially within sai a ode in the pa e be n id c t d a d to m said piece axially within said space. a

A tunabl e twn'd ah se device e mpri i s a cath de, an anod str cture te m d ith re o a ca itie r a g co i ny a o sai c e and ex e d n beyond said. .ca hqde, mean a jacen sai an tructu e f p odu in a ma e ic fi d i h pace b twe n said cathode and anode, a cylindrical piece of conductive material formed withja portion having an increased crosssectional area in juxtaposition to the openings to the anode cavities, a layer of lossy material about the piece immediately behind the enlarged portion, means to support said piece coaxially Within said anode in the space beyond said cathode and to move said piece axially said space.' e

.6. A tunable electron discharge device comprising a cathode, an anode structure formed with resonant cavities arranged coargially about said cathode and extending beyond said cathoda'means adjacent said anode structure for producinga magnetic field in the space between said cathode and anode, a cylindrical piece of conductive material formed with a portion having an increased crosssectional area injuxtaposition to the vopenings to the anode cavities,;a layer of a mixture. of ceramic and metallic material about the piece immediately behind the nlarged portion.

7. A tunable electron discharge device comprising a cathode, an anode structure formed with resonant cavities arranged coaxially about saidcathode'and. extending beyond said cathode, means adjacent said anode structure for producing a magnetic field in the space between said cathode and anodefa cylindrical'piece of conductive material 'fornied'with a portion having an increased crosssectionalarea :located between portions ofilarger cross: sectional area, means to support said piece coaxially with.- in said anode in the .space'beyond .said cathode and to move said piece axiallywithiir said space comprising a threaded extension on said piece'mounted in said anode structure and aflexible diaphragm connecting ,saidpiece and said anode structure. i

8. A tunable electron discharge device comprising a cathode, an anode structure formed with'resonant cavities arranged coaxially about said cathode and extending be yond said cathode, means adjacent said anode structure for producing a magnetic field in the space between said cathode and anodefa cylindrical piece of conductive ma: terial formed with a portion having an increased crosssectional area in juxtaposition to the openings to the anode cavities, means .to support said piece 'coaxially Within said 'an ode'in the space ibeyond said cathode and to movelsaid piece axially within said space comprising a threaded extension .on said'ip'iece mounted in said anode structure and a flexible diaphragm connecting said piece and said anode structure.

(A tunable electron discharge device comprising a cathode, an anede structure formed with resonant cavities arranged coaxially about said cathode and extending beyond' said cathode, means adjacent said anode structure for producing a magnetic field in the space between said cathode and anode, a cylindrical piece of conductive material formed with a portion having an increased crosssectional area in juxtaposition to the openings to the anode cavities, a layer of lossy material about the piece immediately behind the enlarged portion, means to support said piece coaxially Within said anode in the space beyond said cathode and to move said piece axially within said space comprising a threaded extension on said piece mounted in said anode structure and a flexible diaphragm connecting said piece and said anode structure.

10. A tunable electron discharge device comprising a cathode, an anode structure formed with resonant cavities arranged coaxially about said cathode and extending beyond said cathode, means adjacent said anode structure for producing a magnetic field in the space between said cathode and anode, a cylindrical piece of conductive material formed with a portion having an increased crosssectional area in justaposition to the openings to the anode cavities, a layer of a mixture of ceramic and metallic material about the piece immediately behind the enlarged portion, means to support said piece coaxially within said anode in the space beyond said cathode and to move said piece axially within said space comprising a threaded extension on said piece mounted in said anode structure and a flexible diaphragm connecting said piece and said anode structure.

References Cited in the file of this patent UNITED STATES PATENTS 2,408,237 Spencer Sept. 24, 1946 

